Raman Investigations to Identify Corallium Rubrum in Iron Age Jewelry and Ornaments

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Article Raman Investigations to Identify Corallium rubrum in Iron Age Jewelry and Ornaments

Sebastian Fürst 1,†, Katharina Müller 2,†, Liliana Gianni 2,†, Céline Paris 3,†, Ludovic Bellot-Gurlet 3,†, Christopher F.E. Pare 1,† and Ina Reiche 2,4,†,*

1 Vor- und Frühgeschichtliche Archäologie, Institut für Altertumswissenschaften, Johannes Gutenberg-Universität Mainz, Schillerstraße 11, Mainz 55116, Germany; [email protected] (S.F.); [email protected] (C.F.E.P.) 2 Sorbonne Universités, UPMC Université Paris 06, CNRS, UMR 8220, Laboratoire d‘archéologie moléculaire et structurale (LAMS), 4 Place Jussieu, 75005 Paris, France; [email protected] (K.M.); [email protected] (L.G.) 3 Sorbonne Universités, UPMC Université Paris 06, CNRS, UMR 8233, De la molécule au nano-objets: réactivité, interactions et spectroscopies (MONARIS), 4 Place Jussieu, 75005 Paris, France; [email protected] (C.P.); [email protected] (L.B.-G.) 4 Rathgen-Forschungslabor, Staatliche Museen zu Berlin-Preußischer Kulturbesitz, Schloßstraße 1 a, Berlin 14059, Germany * Correspondence: [email protected] or [email protected]; Tel.: +49-30-2664-27101 † These authors contributed equally to this work.

Academic Editor: Steve Weiner Received: 31 December 2015; Accepted: 1 June 2016; Published: 15 June 2016

Abstract: During the Central European Iron Age, more specifically between 600 and 100 BC, red precious corals (Corallium rubrum) became very popular in many regions, often associated with the so-called (early) Celts. Red corals are ideally suited to investigate several key questions of Iron Age research, like trade patterns or social and economic structures. While it is fairly easy to distinguish modern C. rubrum from bone, ivory or shells, archaeologists are confronted with ancient, hence altered, artifacts. Due to ageing processes, archaeological corals lose their intensive red color and shiny surface and can easily be confused with these other light colored materials. We propose a non-destructive multi-stage approach to identify archaeological corals amongst other biominerals used as ornament during the central European Iron Age with emphasis on optical examination and mobile Raman spectroscopy. Our investigations suggest that the noticeably high amount of misidentifications or at least uncertain material declarations existing in museums or even in the literature (around 15%) could be overcome by the proposed approach. Furthermore, the range of different materials is higher than previously expected in archaeological research. This finding has implications for contemporary concepts of social structures and distribution networks during the Iron Age.

Keywords: corals; shells; Raman spectroscopy; biogenic carbonates; carotenoids; polyenes; color fading; material degradation; archaeology

1. Introduction Since Neolithic [1–3] and maybe even in Paleolithic times, 30,000 years ago [4,5], the skeletons of red precious corals (Corallium rubrum (C. rubrum)) were used as jewelry and ornaments. Because of their shiny red color and their dense and hard structure, it is the most demanded coral species in the jewelry industry, which led to an intensive commercial harvesting during the 19th and early 20th century and, in the aftermath, to a decline of its stocks (e.g., [6,7]).

Minerals 2016, 6, 56; doi:10.3390/min6020056 www.mdpi.com/journal/minerals Minerals 2016, 6, 56 2 of 23

Corallium rubrum is a species within the Corallium genus of the Corallidae family and belongs taxonomically to the Anthozoa class within the phylum Cnidaria (e.g., [3,8,9]). The Anthozoa class is divided into three subclasses (Octocorallia, Hexacorallia and Ceriantharia), with the first two as the most important, being determined according to their eight-fold radial symmetry, like C. rubrum, and the third subclass by hexaradial symmetry. A living specimen of C. rubrum consists of three major components: the coral polyps are embedded in a living integument called the coenenchyme, sarcosome or coenosarc, which covers an axial skeleton [1,4,10–13]. In jewelry manufacturing, though, only the calcitic skeleton is of importance, which is why the 0.2–0.3 mm-thin coenosarc is removed prior to processing [1,3]. Unpolished Corallium branches show very characteristic longitudinally-oriented grooves on their surface, as discussed in Section 3.1, deriving from large canals in the coenosarc, which connect the polyps with each other in order to transmit nutrition and information [1,4,14]. The axial skeleton of C. rubrum consists of high-magnesium calcite (HMC) of ca. 88% Ca and compositionally 12% Mg (in number of atoms) together with some minor and trace elements [13,15,16]. The organic matrix (OM) of the axial skeleton of C. rubrum basically consists of acidic proteins and sugars [12]. Additionally, the OM contains small amounts of organic pigments. The biomineralization processes were investigated in several studies (e.g., [17–19]). Allemand and Bénazet-Tambutté [15] have shown that the skeletogenic epithelium in the coenosarc is responsible for the growth of the coral colony (from a few millimeters up to 1–2 cm per year [1,11]) by secretion of calcium carbonate, while the mechanical incorporation of sclerites produced by the scleroblasts only takes place in the apical part of the coral colony, i.e., the 1-cm terminal portion of the branches. However, new investigations of Perrin et al. have shown that the absence of sclerites from the annular part must be reconsidered, as well as the notion that there is no “cement” between the sclerites [17]. The organic pigment comprises various conjugated hydrocarbon chains, which are responsible for the intense red color of the coral. However, the exact structure of these organic compounds is still the subject of discussion. Several studies suggest a coloration due to carotenoids [8,9,11,20–22]. Other works propose unmethylated polyenes [23] and unsubstituted, all trans-polyacetylenic molecules [24], respectively. Several authors think that actually, there is a mixture of pigments each belonging to the same family [25,26]. Karampelas et al. [27] and Bergamonti et al. [28] showed that partially or completely demethylated polyenes with different conjugation lengths seem to be responsible for the colors of the species of the genus Corallium and most of the marine shells, whereas the red aragonitic hydroids of the species Stylaster roseus are colored by carotenoids, similar to those found in canthaxanthin. Raman studies on mollusks and their pearls revealed at least four and up to nine pigments in the same pearl, even in homogeneously-pigmented zones [24,29]. Chain lengths ranging from 6–14 conjugated double bonds deriving from carotenoids with unmethylated polyacetylenic backbones of various conjugation lengths were reported to be responsible for the colors. The identification of C. rubrum seems to be fairly easy for modern pieces, but can be difficult for those from archaeological records. Corallium rubrum loses its intense red color and shiny surface structure due to not yet fully understood degradation processes, which is why it can easily be mistaken for light colored materials, like bone, ivory, shells or even limestone, also used for jewelry in Central Europe during prehistoric times. Moreover, the corals and other biominerals were usually used as small polished studs or pearls (average diameter of only 0.5–1 cm), which have only small, concealed surface characteristics. It is therefore not surprising that 80 years ago, many researchers did not know that white inlays could have been made from red coral. Today, however, there is a slight tendency to declare all light colored materials as C. rubrum. Attempts have repeatedly been made to identify the raw material of the frequently-encountered and today whitish jewelry inlays using different analytical methods, as for instance X-ray fluorescence (XRF) [30–32], X-ray diffraction (XRD), isotope analysis, as well as electron microprobe (EMP) [32,33], micro-computed X-ray tomography (µ-CT) [34] and Raman spectroscopy [35]. These studies help toward better knowledge of the biominerals used for the manufacturing of precious objects. Minerals 2016, 6, 56 3 of 23

Schrickel et al. [35–38] recently used micro-X-ray tomography (micro-CT), digital microscopy, full-field micro-XRF using the X-ray-color camera, Raman spectroscopy and XRD to investigate mainly late Iron Age coral ornamented fibulae (“Mitteldeutsche Korallenfibeln”) from Central Germany. Based on the MgCO3 contents and specific structural features, the authors conclude that most of the materials used to ornament the fibulae were nummulites or other Foraminifera, and not C. rubrum. During the Central European Iron Age, more specifically between 600 and 100 BC, red precious corals (C. rubrum) became very popular in many regions, often associated with the so-called (early) Celts. Red corals are ideally suited to investigate several key questions of Iron Age research [39–44]. As a prestigious long-distance trade good, C. rubrum can change the social status of a grave or site and, yet, its whole interpretation. On a macro scale, the distribution of coral finds help to gain a better understanding of trade patterns, communication routes and Iron Age social structures. It is therefore very instructive to generate an approximation of the extent of the actual use of coral and to find out which other materials have been applied as jewelry ornaments at which time. A better identification of the raw material used for the manufacturing of ancient objects will also contribute to the reconstruction of the original appearance of the objects. However, before these questions can be answered and interpreted, the first objective is to establish an investigation protocol for biominerals from archaeological sites to unambiguously identify C. rubrum on archaeological artifacts. Since most archaeological institutions do not have access to much scientific equipment and do not have the time for detailed investigations, this identification procedure must meet a few requirements. It has to be both efficient and reliable, and of course, the analyses must be non-invasive. This is why the proposed identification protocol focuses on visual and mesoscopic observations, as well as Raman spectroscopy. Since there are so many alleged coral objects (several thousand of individual pieces), the proposed approach must be flexible enough in order to study large collections with reasonable means. Studying the distribution of raw materials over large areas and time spans requires their reliable determination on representative collections. On basis of this methodology, a representative selection of archaeological artifacts was studied in order to test earlier attributions found in the literature or in museum inventories.

2. Materials and Methods

2.1. Materials In order to establish a reliable catalogue of the structural features of archaeological corals, two successive steps are necessary. Ancient unprocessed corals and mollusks, which can be clearly determined as such, are the starting point. These are necessary to identify structural features caused by alterations, such as changes in color or surface texture. Secondly, a sufficiently large collection of recent corals and mollusks has been investigated as reference material (Table1). It was considered important to use taxonomically-reliable specimens from the collections of natural history museums to avoid misinterpretation by using imitations of precious corals purchased on the free market [3,11]. Attention was also paid to investigate as many different coral and mollusk species as possible, e.g., white coral, organ pipe coral (Tubipora musica) or Scleractinia. Furthermore, other species of the genus Corallium from the Indo-Pacific were examined [45]. Even fossil precious corals (6–7.2 million years old) and samples from the Atlantic Ocean were studied. Furthermore, care was taken to ensure that the coral samples of C. rubrum came from as many different Mediterranean regions as possible. Nowadays, C. rubrum mainly occurs between North Africa, Spain, France and Italy, but it can also be found in the adjacent Atlantic Ocean between the Algarve and Cape Verde [3,9,46,47]. Since both the archaeological coastal finds [48] and the written historical location information [49] match with the current regions of coral harvesting, there seems to have been hardly any shifts in terms of provenance since prehistoric times. Minerals 2016, 6, 56 4 of 23

Table 1. List of reference objects, which have been analyzed by Raman spectroscopy and/or by XRF. The number of obtained Raman spectra is signiﬁcantly higher, because some objects showed several interesting and different-looking areas that were examined.

Species * Origin * Analyzed Objects Corals (Anthozoa) French coast (Marseille), Greece, Italy (Sardinia, Corallium rubrum (red precious coral) Sicily, Tyrrhenian Sea), Malta, Spain (La Escala), 19 Portugal (Algarve) Corallium rubrum (Sciacca coral) Italy (Sicily, Sciacca) 1 Corallium sp., deep sea Portugal (Azores, 1557 m depth) 1 Corallium sp./konojoi(?) (white coral) Japan 1 Corallium sp. (white coral) Unknown 1 Corallium sp., Pleistocene Italy (Sicily, Augusta) 1 Corallium sp., Miocene, Tortonian Spain (Mazarrón) 1 11.6–7.2 million years Corallium secundum (red Momo coral) Taiwan 1 Corallium elatius (porous Mushi coral) Philippines, Nepal 2 Tubipora musica (organ pipe coral) Unknown, Red Sea or Indo-Paciﬁc 1 Scleractinia Unknown 2 Shells (Bivalvia) Greece (Morea), Italy (Trieste, Sardinia); Egypt Spondylus gaederopus (thorny/spiny oysters) 5 (Red Sea, Hurghada) 1 Spondylus barbatus (thorny/spiny oysters) Indo-Paciﬁc 1 Spondylus sp. (thorny/spiny oysters) United Arab Emirates 1 Sea Snails (Gastropoda) Cypraecassis testiculus Dominican Republic, Cape Verde (Sao Vicente) 1 2 (reticulated cowrie helmet) Semicassis saburon (helmet/bonnet snail) Italy (Naples) 1 * The information on the species and origin is taken directly from the collections of the museums that kindly provided the objects: Museum of Natural History Mainz, Institute of gemstone research Idar-Oberstein and the Naturmuseum Senckenberg Frankfurt; 1 XRF only.

It has to be noted that the number of potential raw materials, at least for the Iron Age period, is much smaller than today, since the archaeological record suggests that there has been no importation of corals or mollusks from farther away than the seas adjacent to Central Europe (Mediterranean Sea, Atlantic Ocean, North and Baltic Sea). As a consequence, all coral and mollusk species from the Indo-Paciﬁc or the tropical areas of the Atlantic Ocean can be excluded when it comes to identiﬁcation of Western European archaeological artifacts. A total of 95 archaeological artifacts (see Table S1) and 41 recent corals and mollusks have been analyzed using Raman spectroscopy. Furthermore, 50 recent corals and mollusks and two coral inlays from the Iron Age gold helmet of Agris (département Charente, France) have been analyzed by XRF.

2.2. Methods

2.2.1. Microscopy In order to investigate the characteristic structural features, the objects were examined using a digital Leica DVM 2000 microscope (Leica Microsystems GmbH, Wetzlar, Germany) with an optical zoom range from 50ˆ–400ˆ magniﬁcation and an analog Zeiss Microscope (Carl Zeiss AG, Oberkochen, Germany) with an attached digital camera and an optical zoom range from 16ˆ–80ˆ magniﬁcation. Minerals 2016, 6, 56 5 of 23

Minerals 2016, 6, 56 5 of 22 2.2.2. Raman Spectroscopy 2.2.2. Raman Spectroscopy Raman spectroscopy allows detecting both the mineral part and the organic pigments of red coral. ResonanceRaman or near-resonance spectroscopy allows Raman detecting scattering both is exceptionallythe mineral part sensitive and the toorganic both pigmentpigments groups,of red the carotenoidscoral. Resonance and polyenes, or near even-resonance in very Raman low concentrations scattering is exce [27ptionally]. Numerous sensitive studies to both on pigment red corals or red coloredgroups, marinethe carotenoids shells were and carriedpolyenes, out even using in Ramanvery low spectroscopy concentrations [8 ,[27].11,18 Numerous,20–28]. Kupka studieset on al. [26] red corals or red colored marine shells were carried out using Raman spectroscopy [8,11,18,20–28]. state that a very small amount of organic matter is able to change the color of this coral from white to Kupka et al. [26] state that a very small amount of organic matter is able to change the color of this pink, leading to the conclusion that the Raman activity of such ‘chromophores’ is very high. coral from white to pink, leading to the conclusion that the Raman activity of such ‘chromophores’ is Thevery Ramanhigh. analyses were performed using various spectrometers (bench-top or portable) with various excitationThe Raman wavelengths analyses were and performed under differentusing variou experimentals spectrometers conditions (bench-top to or determine portable) with the most suitablevarious settings excitation (for wavelengths a summary and of the under already different used experimental spectrometers conditions and measurementto determine the conditions, most see Tablesuitable S2). settings (for a summary of the already used spectrometers and measurement conditions, see InTable Raman S2). analyses of corals and mollusks, the choice of the best-suited laser wavelength depends on whetherIn theRaman colorants analyses or of the corals mineral andphase mollusks, are thethe focuschoice of of interest the best-s (seeuited Figure laser1). wavelength In general, the higherdepends the wavelength, on whether the the better colorants are theor the mineral mineral phases phase detected,are the focus especially of interest with (see 785 Figure nm, because1). In of general, the higher the wavelength, the better are the mineral phases detected, especially with the ﬂuorescence reduction with this wavelength and the out-of-resonance conditions for the pigment. 785 nm, because of the fluorescence reduction with this wavelength and the out-of-resonance The lower the wavelength, the better are the colorants detected, especially with 488 and 532 nm, conditions for the pigment. The lower the wavelength, the better are the colorants detected, becauseespecially for these with organic488 and 532 molecules nm, because have for a these strong organic resonance molecules effect, have which a strong enhances resonance their effect, Raman diffusionwhich over enhances the ﬂuorescence their Raman emission diffusion and over even the fluo mineralrescence signature. emission The and 632.8-nm even mineral wavelength signature. turned out toThe be 632.8-nm the best wavelength compromise turned to detect out to both be the features best compromise at the same to detect time; however,both features not at with the same the same intensitytime; (sensitivity) however, not as with with the the same “ideal” intensity wavelengths (sensitivity) for as each with feature, the “ideal” respectively. wavelengths for each feature, respectively.

FigureFigure 1. Raman 1. Raman spectrum spectrum of the of the same same spot spot of aofC. a rubrumC. rubrumsample sample using using 785 785 nm nm (blue) (blue) and and488 488 nmnm (red), indicating(red), theindicating visibility the of visibility different of features different of featuresC. rubrum of .C. Abbreviations: rubrum. Abbreviations: T = translation; T = translation;L = libration ; L = libration; ν4 = in-plane bending; ν1 = symmetric stretching. ν4 = in-plane bending; ν1 = symmetric stretching. Figure 1 illustrates the different spectra obtained by a 785-nm and a 488-nm excitation under Figurelaboratory1 illustrates conditions. the Among different the spectrafeatures obtainedof the high by magnesium a 785-nm calcite, and a 488-nmthe symmetric excitation (CO3) under2− 2´ laboratorystretching conditions. mode (ν1) Amongis the main the band features and occurs of the at high around magnesium 1092 cm−1. Only calcite, two the bands symmetric appear in (COthe 3) ´1 stretchinglattice modevibration (ν1) region is the mainof the band high and magnesium occurs at calcite around spectrum: 1092 cm the. Only libration two bandsat 290 appearand the in the latticetranslation vibration at region 161 cm of−1 the. The high ν4 in-plane magnesium bend calciteis present spectrum: at 720 cm the−1. librationSeveral studies at 290 [18,50,51] and the translation have shown´1 that the peak positions of HMC shift towards´ higher1 wavenumber regions in the Raman at 161 cm . The ν4 in-plane bend is present at 720 cm . Several studies [18,50,51] have shown that −1 the peakspectrum. positions Thus, of the HMC peak shift positions towards of higherpure ca wavenumberlcite are at around regions 156, in 283, the Raman713 and spectrum. 1086 cm , Thus, respectively; Eg,ext modes for the two first bands (T, L) and Eg,int (ν4) and A1g,int (ν1) modes for the the peak positions of pure calcite are at around 156, 283, 713 and 1086 cm´1, respectively; E modes following. The aragonite spectrum of many mollusks differs from the calcite spectrum especiallyg,ext in ν ν for thethe two lattice ﬁrst vibration bands (T,region L) andwith E bandsg,int ( of4) medium and A1g,int intensity( 1) modes at 207 and for the154 following.cm−1 plus other The weak aragonite spectrumintensity of many bands, mollusks as well as differs a double from band the for calcite the ν spectrum4 planar bending especially (Eg,int in) at the 702 lattice and 706 vibration cm−1 (see region ´1 withdiscussion bands of mediumbelow). intensity at 207 and 154 cm plus other weak intensity bands, as well as a ´1 double band for the ν4 planar bending (Eg,int) at 702 and 706 cm (see discussion below). Concerning the colorants, the two main bands with the highest intensity are the stretching ´1 modes of the C=C double bonds (ν1) at around 1520 cm and the C–C single bonds (ν2) at around Minerals 2016, 6, 56 6 of 22

Concerning the colorants, the two main bands with the highest intensity are the stretching Minerals 2016, 6, 56 6 of 23 modes of the C=C double bonds (ν1) at around 1520 cm−1 and the C–C single bonds (ν2) at around 1130 ± 15 cm−1. Several studies have shown that Raman spectra of white areas on recent corals, mollusks1130 ˘ or15 natural cm´1. pearls Several do studies not show have the shown characteristic that Raman bands spectra of organic of whitedye or areas only onwith recent very corals,low intensitiesmollusks (as or discussed natural pearls belowdo in Section not show 4.2) the [11,18,26,29]. characteristic Observations bands of organic have pointed dye or out only that with the veryν2 vibrationlow intensities enables (as a discusseddistinction below between in Section unsubstituted 4.2)[11,18 ,polyenes26,29]. Observations that can be have found pointed at about out that 1130 ± 15 cm−1, while carotenoid bands appear at approximately 1155 ± 15 cm−1 [24,25,52]. the ν2 vibration enables a distinction between unsubstituted polyenes that can be found at about 1130A ˘part15 of cm the´1, whileanalyses carotenoid was carried bands out appear “on-site” at approximately in the museums 1155 using˘ 15 cm portable´1 [24, 25equipment,,52]. which doesA part not of provide the analyses the same was carried spectral out resolution “on-site” inas thelaboratory museums spectrometers using portable (see equipment, Table S2). which In thosedoes cases, not provide it is necessary the same to spectral use a 785-nm resolution exci astation laboratory to determine spectrometers the mineral (seeTable phase S2). and In a those 532-nm cases, excitationit is necessary for the tocolorant use a 785-nm detection excitation (which are to determine the laser lines the mineralavailable phase for ou andr portable a 532-nm instruments). excitation for the colorant detection (which are the laser lines available for our portable instruments). 3. Results 3. Results 3.1. Optical Differences between the Surface Structures of Ancient and Recent Precious Corals 3.1. Optical Differences between the Surface Structures of Ancient and Recent Precious Corals From the end of the seventh until the middle of the fifth century BC, almost unprocessed C. rubrumFrom was the quite end ofpopular, the seventh especially until theworn middle as neckla of theces, ﬁfth pinheads century BC,or pendants. almost unprocessed The naturalC. state rubrum allowswas quitea comprehensive popular, especially examination worn asof necklaces,the structural pinheads features. or pendants.A representative The natural selection state of allows Iron a Agecomprehensive artifacts, which examination are clearly of identifiable the structural as Corallium features., A is representative shown in Figure selection 2. Although of Iron Agethese artifacts, four exampleswhich are look clearly more identiﬁableor less faded, as Coralliumat least not, is as shown red as in modern Figure 2red. Although precious these corals, four all examplesof them still look revealmore small or less areas faded, of at leastred notcolor, as redsometimes as modern only red preciousvisible by corals, microscopical all of them stillobservations reveal small (see areas Figureof red 3c,f). color, sometimes only visible by microscopical observations (see Figure3c,f).

(a)

(b)

Figure 2. Cont.

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Minerals 2016, 6, 56 7 of 23 Minerals 2016, 6, 56 7 of 22

(c)

(d)

Figure 2. Examples of almost unprocessed corals from the Iron Age (~620–450 BC) in southwestern Germany. (a) Necklace from Tomb 122 of the Magdalenenberg tumulus near Villingen-Schwenningen (Schwarzwald-Baar-Kreis).(d) A detail is shown in Figure 3e. (b) Pectoral necklace from a tumulus near Allensbach-Kaltbrunn (Landkreis Konstanz). (c) Piece of oddly Figure 2. Examples of almost unprocessed corals from the Iron Age (~620–450 BC) in southwestern circular-shapedFigure 2. Examples coral ofbranch almost from unprocessed Burial 6 of corals the Hohmichele from the Iron tumulus Age (~620–450 near Altheim-Heiligkreuztal. BC) in southwestern Germany. (a) Necklace from Tomb 122 of the Magdalenenberg tumulus near (dGermany.) Necklace (a of) Necklace single coral from pieces Tomb and 122 egg-sized of the Magdalenenberg bulbs, composed tumulus of several near lamellae Villingen-Schwenningen from the burial Villingen-Schwenningen (Schwarzwald-Baar-Kreis). A detail is shown in Figure 3e. (b) Pectoral of(Schwarzwald-Baar-Kreis). a young woman at Esslingen-Sirnau. A detail is shown in Figure3e. ( b) Pectoral necklace from a tumulus near necklaceAllensbach-Kaltbrunn from a tumulus (Landkreis near Allensba Konstanz).ch-Kaltbrunn (c) Piece of oddly(Landkreis circular-shaped Konstanz). coral (c) branch Piece fromof oddly Burial Acircular-shaped6 ofcomparison the Hohmichele coralof the branch tumulus structural from near Burial Altheim-Heiligkreuztal.similarities 6 of the Hohmicheleof recent and (tumulusd) Necklacearchaeological near of Altheim-Heiligkreuztal. single corals, coral piecesas shown and in (d) Necklace of single coral pieces and egg-sized bulbs, composed of several lamellae from the burial Figureegg-sized 3, allows bulbs, the identification composed of several of corals, lamellae even from in the the case burial of of small a young studs. woman at Esslingen-Sirnau. of a young woman at Esslingen-Sirnau.

A comparison of the structural similarities of recent and archaeological corals, as shown in Figure 3, allows the identification of corals, even in the case of small studs.

(a) (b)

Figure 3. Cont.

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(e)

(g)

(f)

FigureFigure 3. 3.Surface Surface structures structures of of ancient ancient corals corals and and similarities similarities with with recent recent reference reference objects. objects. (a ()a Inner) Inner sideside of of three three lamellae lamellae of a coralof a beadcoral from bead a femalefrom gravea female in Ditzingen-Schöckingen grave in Ditzingen-Schöckingen (Landkreis Ludwigsburg).(Landkreis Ludwigsburg). (b) Corallium (b rubrum) Coralliumfrom rubrum an area from near an Marseille area near with Marseille longitudinal with crenulationlongitudinal coveringcrenulation the stem,covering resulting the stem, in a resulting concentric, in wavya conc structureentric, wavy in the structure cross section. in the cross (c) Belt section. chain ( fromc) Belt Bussy-le-Châteauchain from Bussy-le-Château (département Marne),(département showing Marne) three, concentric showing featuresthree concentric of more intense features color of thanmore theintense pale center color ofthan the piece,the pale indicating center of the the medullar piece, indi regioncating of a cross-sectionthe medullar of regionC. rubrum of a; seecross-section (Figure3g). of (dC.) Recent rubrumC.; see rubrum (Figurefrom 3g). the ( Straitd) Recent of Messina C. rubrum also showingfrom the partsStrait of of faded Messina color. also (e) Coralshowing bead parts from of Tombfaded 122 color. of the (e) Magdalenenberg Coral bead from tumulus; Tomb 122 detail of the of FigureMagdalenenberg2a. ( f) Detail tumulus; of a partially detail redof Figure area of 2a. a C.(f rubrum) Detailpiece of a frompartially Somme-Tourbe red area of a “la C. Bouvandeau” rubrum piece infrom the Somme-Tourbe Champagne. (g )“la Section Bouvandeau” of a Corallium in the rubrumChampagne.colony after(g) Section decalciﬁcation of a Corallium and staining rubrum colony [47,53]. after decalcification and staining [47,53].

AThe comparison most characteristic of the structural structure similarities of the C. rubrum of recent skeleton and archaeologicalcan be seen both corals, on the as inner shown side in of Figurethree3 lamellae, allows theof an identiﬁcation originally egg-sized of corals, evencoral inbead the from case ofa smallfemale studs. grave in Ditzingen-Schöckingen (LandkreisThe most Ludwigsburg) characteristic (Figure structure 3a), of as the wellC. rubrumas on a skeletonrecent C. canrubrum be seen stem both from on the the Mediterranean inner side of threeSea lamellaeoff Marseille of an originally(Figure 3b). egg-sized This longitudinally-crenulated coral bead from a female gravesurface in Ditzingen-Schöckingenis a distinctive sign of (LandkreisC. rubrum Ludwigsburg)[1,3,4,13,17]. As (Figure seen 3ona), asthe well recent as onspecimen, a recent theseC. rubrum groovesstem cause from a the wavy Mediterranean edge of the Seacross-section. off Marseille The (Figuremicroscope3b). picture This longitudinally-crenulated of the belt chain from Bussy-le-Châte surface isau a distinctive(département sign Marne, of C.France) rubrum (Figure[1,3,4,13 3c),17 ].shows As seen a slightly on the lighter recent specimen,center of irregular these grooves shape cause surrounded a wavy by edge concentric of the cross-section.structures indicating The microscope the medullar picture region of the of belt a chaincross-section from Bussy-le-Château of C. rubrum, as (départementindicated by the Marne, grey France)area in (Figure Figure3c) 3g. shows The aspecimen slightly lighter in Figure center 3d of irregulardemonstrated shape that surrounded recent corals by concentric from the structuresMediterranean indicating Sea the(from medullar the Strait region of Messina of a cross-section in this case) of C.can rubrum also be, as partly indicated uncolored. by the greyWhile area the inrecent Figure coral3g. Thein Figure specimen 3b exhibits in Figure only3d demonstrateda wavy edge in that the recent cross-section, corals from the thecoral Mediterranean beads from the Seaalready (from mentioned the Strait of Tomb Messina 122 inof thisthe case)Magdalenenberg can also be partlytumulus uncolored. in Baden-Württemberg While the recent (Figure coral in3e) Figureand 3ab small exhibits inlay only of a wavya shoe edge knob in thefrom cross-section, Somme-Tourbe the coral “la beadsBouvandeau” from the alreadyin the mentionedChampagne Tomb(Figure 122 3f) of present the Magdalenenberg a circular domain tumulus composed in Baden-Württemberg of concentric rings. (Figure The 3differencee) and a small between inlay these of atwo shoe lies knob in the from surface Somme-Tourbe structures: “la the Bouvandeau” Somme-Tourbe in piece the Champagne shows in a (Figurestill colored3f) present area (on a circular the right domainside of composed the photo) of a concentric conspicuous rings. color The differencebanding. A between similar these annular two liesdomain in the can surface be seen structures: on the Magdalenenberg piece, but not as a color banding, rather as small gaps. These concentric corrugated features correspond to the growth rings of a stained cross-section of C. rubrum, as shown on

Minerals 2016, 6, 56 9 of 22

Figure 3g [1,3,4,13,17,45,47,53]. The examples from Ditzingen-Schöckingen (Figure 3a), Bussy-le-Château (Figure 3c) and especially from Somme-Tourbe (Figure 3f) illustrate a further interesting structural correlation: the more color that can be seen on an artifact, the more lustrous Mineralsand smooth2016, 6 is, 56 the surface. 9 of 23 Larger and rounder shapes of modern C. rubrum usually derive from the sockets of a Corallium colony. In this case, the C. rubrum pieces are called “foam coral” because of their porous structure the Somme-Tourbe piece shows in a still colored area (on the right side of the photo) a conspicuous (see Figure 4a) [3]. color banding. A similar annular domain can be seen on the Magdalenenberg piece, but not as a color banding, rather as small gaps. These concentric corrugated features correspond to the growth rings of 3.2. Remains of the Former Coloring a stained cross-section of C. rubrum, as shown on Figure3g [ 1,3,4,13,17,45,47,53]. The examples from Ditzingen-SchöckingenMany archaeological (Figure pieces3a), reveal Bussy-le-Château still colored ar (Figureeas (Figures3c) and 3a especially and 4), most from often Somme-Tourbe at formerly (Figurewell-protected3f) illustrate parts, a as further seen in interesting Figure 4a structural(see also Figure correlation: 3a), where the morethe lamellae color that of the can egg-sized be seen on coral an artifact,beads are the still more red lustrous on the andsides, smooth because is theother surface. lamellae were originally attached. In most cases, the fadingLarger of the and color rounder progresses shapes from of modern the outsideC. rubrum to theusually inside. deriveFurthermore, from the coral sockets is often of a still Corallium red at colony.points that In thisare case,in direct the C.contact rubrum withpieces metal, are i.e. called, around “foam rivets coral” (Figure because 4b) ofor their at the porous edge structureof inlays (see(Figure Figure 3f). 4a) [3].

(a) (b)

Figure 4. (a) Detail of the almost egg-sized coral bead from the female burial of Esslingen-Sirnau; see Figure2 2d;d; ( b(b)) detail detail of of another another almost almost egg-sized egg-sized coral coral bead bead from from Ditzingen-Schöckingen, Ditzingen-Schöckingen, where where a small a redsmall part red around part around the bronze the bronze pin is visible.pin is visible.

3.2.3.3. Identification Remains of the of Former Ancient Coloring Corals: Case Studies ManyThe following archaeological case piecesstudies reveal were still chosen colored to areas demonstrate (Figures3 ahow and 4),the most optical often and at formerly Raman well-protectedinvestigations of parts, today as light seen colored in Figure biominerals4a (see also on Figure archaeological3a), where artifacts the lamellae help to of inform the egg-sized about the coralnature beads of the are materials still red used on the to sides,decorate because the objects other in lamellae ancient were times. originally attached. In most cases, the fadingThe analyses of the color of the progresses three light from colored the outside pieces to of the a small inside. fibula Furthermore, (Figure 5) coral from is a often rich stillfemale red atgrave points of the that second are in directhalf of contact the fifth with century metal, BCi.e. ,from around Schwieberdingen rivets (Figure4 b)“Wartbiegel” or at the edge in ofnorthern inlays (FigureBaden-Württemberg3f). in Germany yielded interesting results concerning the usage of different materials on one artifact, the connections to long distance trade objects and the social structures at 3.3.that Identificationtime. Nowadays, of Ancient the preservation Corals: Case Studies conditions and colors of the three pieces on this very small (only 3 cm long) iron fibula differ from each other. The small bead at the reversed foot of the fibula is The following case studies were chosen to demonstrate how the optical and Raman investigations almost completely covered by layers of brown corrosion material and has an irregularly-formed of today light colored biominerals on archaeological artifacts help to inform about the nature of the surface. The largest piece on the bow at the center of the fibula is placed on a thin gold foil and is materials used to decorate the objects in ancient times. fixed by a small golden rivet. Although this cabochon is partially broken and the surface is also The analyses of the three light colored pieces of a small fibula (Figure5) from a rich female irregular, it still has pinkish and orange hues. The small bead on the spring in the front of the fibula grave of the second half of the fifth century BC from Schwieberdingen “Wartbiegel” in northern has a light and intact surface texture, but with a few small brown spots of corrosion products. The Baden-Württemberg in Germany yielded interesting results concerning the usage of different materials spectra of the pieces from the foot and bow show features of calcite with a (CO3)2− main band at on one artifact, the connections to long distance trade objects and the social structures at that time. 1086 cm−1 and two minor bands at 282 and 713 cm−1. The wavenumber position is only slightly higher Nowadays, the preservation conditions and colors of the three pieces on this very small (only 3 cm than that of biogenic calcite and is in the lower region of the expected value for HMC as long) iron fibula differ from each other. The small bead at the reversed foot of the fibula is almost incorporated in modern C. rubrum (see Figure 1; [18,50,51]). The characteristic ν1 and ν2 bands, completely covered by layers of brown corrosion material and has an irregularly-formed surface. The representing the unmethylated polyenic pigments at 1518 and 1130 cm−1, of the still orange colored largest piece on the bow at the center of the fibula is placed on a thin gold foil and is fixed by a small bead from the bow are very intense and as high as the (CO3)2− band, even with a 632.8-nm excitation. golden rivet. Although this cabochon is partially broken and the surface is also irregular, it still has pinkish and orange hues. The small bead on the spring in the front of the fibula has a light and intact surface texture, but with a few small brown spots of corrosion products. The spectra of the pieces 2´ ´1 from the foot and bow show features of calcite with a (CO3) main band at 1086 cm and two minor bands at 282 and 713 cm´1. The wavenumber position is only slightly higher than that of biogenic Minerals 2016, 6, 56 10 of 22

Two more pigment bands at 1020 and 1294 cm−1 are visible. The combination of a broad calcite band together with intense pigment peaks offers a high probability of C. rubrum.

MineralsIn 2016contrast,, 6, 56 the piece from the spring of the fibula yielded a spectrum with a sharp ν1 calcite10 peak of 23 at 1086 cm−1, a band at 283 cm−1, assigned to the rotational (Eg) mode, and a weak band at 713 cm−1 of the internal Eg corresponding to the in-plane bending modes (ν4), but without any calcitepigment-related and is in thefeatures. lower region of the expected value for HMC as incorporated in modern C. rubrum (see FigureAs mentioned1;[ 18,50 ,above,51]). The the characteristic smaller full widthν1 and atν half2 bands, maximum representing (FWHM) the of unmethylated the calcium carbonate polyenic pigmentsband at 1086 at 1518 cm and−1 of 1130the spring cm´1, ofbead the stillin comparison orange colored to the bead other from two the pieces bow are may very result intense in andless 2´ asMgCO high3 as[18,51], the (CO which3) is,band, together even with ath 632.8-nme pigment excitation. features, characteristic Two more pigment for C. rubrum bands at. Therefore, 1020 and 1294the spring cm´1 beadare visible. is likely The to combinationhave been made of a broadof an almost calcite pure, band non-colored together with calcite, intense maybe pigment marble peaks or offerssome kind a high of probabilityshell. The dense of C. structure rubrum. of the material underlines this suggestion (Figure 5, top right).

Figure 5.5. RamanRaman spectra spectra of of the the three three light colored pi pieceseces of the fibulafibula fromfrom SchwieberdingenSchwieberdingen “Wartbiegel”,“Wartbiegel”, taken with with 632.8 632.8 nm. nm. The The pieces pieces from from the the foot foot (green) (green) and and from from the thebow bow (red) (red) of the of thefibula fibula clearly clearly show show calcite calcite and andpolyene polyene features, features, indicating indicating C. rubrumC. rubrum. In .contrast, In contrast, the thepiece piece from from the spring of the fibula (blue) yields a spectrum with a sharp CaCO3 band at 1086 cm´−11, but no the spring of the fibula (blue) yields a spectrum with a sharp CaCO3 band at 1086 cm , but no pigment bands.

While the Raman measurements of the small iron fibula from Schwieberdingen “Wartbiegel” In contrast, the piece from the spring of the fibula yielded a spectrum with a sharp ν calcite demonstrated that sometimes, coral was combined with other materials on one object, another1 iron peak at 1086 cm´1, a band at 283 cm´1, assigned to the rotational (E ) mode, and a weak band fibula of the same type, only with a bronze sheet instead of a gold foil, gis an example that even one at 713 cm´1 of the internal E corresponding to the in-plane bending modes (ν ), but without any single pearl could be made outg of two different materials. This piece was also found4 in a rich female pigment-related features. burial and dates to the same period (450–400 BC), but this time, ca. 90 km further northwest in As mentioned above, the smaller full width at half maximum (FWHM) of the calcium carbonate Ilvesheim “Weingärten” near Mannheim. band at 1086 cm´1 of the spring bead in comparison to the other two pieces may result in less Of the original seven decorative elements of the iron fibula, the remaining three have been MgCO [18,51], which is, together with the pigment features, characteristic for C. rubrum. Therefore, investigated3 by Raman spectroscopy. The bead on the foot (Figure 6) already showed typical optical the spring bead is likely to have been made of an almost pure, non-colored calcite, maybe marble or signs of C. rubrum, which could be confirmed by the Raman spectrum. The bands labeled in red in some kind of shell. The dense structure of the material underlines this suggestion (Figure5, top right). Figure 6 also indicate that this pearl had originally been red. While the Raman measurements of the small iron fibula from Schwieberdingen “Wartbiegel” Today, the bead on the spring of the fibula has a reddish-brown color. The material used demonstrated that sometimes, coral was combined with other materials on one object, another iron (Figure 7) could not be identified yet, since the obtained spectrum neither corresponds to calcium fibula of the same type, only with a bronze sheet instead of a gold foil, is an example that even carbonates, like corals and mollusks, nor to any kind of apatite-containing material, like bone or one single pearl could be made out of two different materials. This piece was also found in a rich ivory. The presence of amorphous carbon in the region 1580–1600 cm−1 with a smaller band at about female burial and dates to the same period (450–400 BC), but this time, ca. 90 km further northwest in 1458 cm−1 indicates a contamination or residues of organic material. This piece expands the range of Ilvesheim “Weingärten” near Mannheim. materials and shades employed in Iron Age jewelry, since it can be assumed that the original hue Of the original seven decorative elements of the iron fibula, the remaining three have been came close to the current one and that it looked neither like ivory nor like coral. investigated by Raman spectroscopy. The bead on the foot (Figure6) already showed typical optical signs of C. rubrum, which could be confirmed by the Raman spectrum. The bands labeled in red in Figure 6 also indicate that this pearl had originally been red. Minerals 2016, 6, 56 11 of 23 Minerals 2016, 6, 56 11 of 22

Minerals 2016, 6, 56 11 of 22

Figure 6.6.Ilvesheim Ilvesheim “Weingärten”; “Weingärten”; Raman Raman spectrum spectrum of foot of bead foot of smallbead ironof ﬁbula.small Theiron characteristicfibula. The characteristic ν1 and ν2 frequencies of the pigments at 1520´1 and 1129 cm−1 are still very intense. ν1 and ν2 frequencies of the pigments at 1520 and 1129 cm are still very intense. Red = pigments; blackRed = =pigments; calcite. black = calcite.

Today, the bead on the spring of the ﬁbula has a reddish-brown color. The material used (Figure7) could not be identiﬁed yet, since the obtained spectrum neither corresponds to calcium carbonates, like corals and mollusks, nor to any kind of apatite-containing material, like bone or ivory. The presence of amorphous carbon in the region 1580–1600 cm´1 with a smaller band at about 1458 cm´1 indicates a contaminationFigure 6. Ilvesheim or residues “Weingärten”; of organic material.Raman Thisspectrum piece expandsof foot bead the range of small of materials iron fibula. and shades The employedcharacteristic in Iron ν1 and Age ν jewelry,2 frequencies since itof can the be pigments assumed at that 1520 the and original 1129 hue cm came−1 are closestill tovery the intense. current oneRed and= pigments; that it looked black neither= calcite. like ivory nor like coral.

Figure 7. Ilvesheim “Weingärten”; Raman spectrum of the spring bead of the small iron fibula (see Figure 8). The presence of amorphous carbon in the region 1580–1600 cm−1 with a smaller band at about 1340 cm−1 indicates a contamination or residues of organic material.

Particularly surprising were the spectra of the only 0.6-cm small bead of the bow (Figure 8): first, the material was identified as coral (Figure 9), but then, however, further measurements of the supposedly same piece surprisingly yielded several spectra of apatite from different areas of the material (Figure 10), which is generally associated with bone or ivory. A subsequent microscopic examination revealed an extremely small amount of coral adhering to a base made of apatite-containing material (Figure 8, below). However, due to poor preservation, it cannot be clarifiedFigureFigure 7. whether Ilvesheim 7. Ilvesheim a lack “Weingärten”; “Weingärten”; of coral had Raman to Raman be concea spectrum spectrumled or ofof whether thethespring spring a beadred-white bead of of the the smallcontrast small iron ironwas ﬁbula intended.fibula (see (see FigureFigure 8). 8The). The presence presence of of amor amorphousphous carbon inin thethe region region 1580–1600 1580–1600 cm cm´1 −with1 with a smaller a smaller band band at at aboutabout 1340 1340 cm− cm1 indicates´1 indicates a contamination a contamination or or residues residues of of organic organic material.material.

Minerals 2016, 6, 56 12 of 23 Minerals 2016, 6, 56 12 of 22

Minerals 2016, 6, 56 12 of 22

FigureFigure 8. Small 8. Small iron iron fibula ﬁbula from from Ilvesheim Ilvesheim “Weingärten”. “Weingärten”. TheThepicture picture below below shows shows the the detail detail of the ofbead the bead on theon bow the bowwith with the microscopic the microscopic coral coral piece piece adhering adhering to toa base a base made made of of apatite-containing apatite-containing material. material.

Particularly surprising were the spectra of the only 0.6-cm small bead of the bow (Figure8): first, the material was identified as coral (Figure9), but then, however, further measurements of the supposedly same piece surprisingly yielded several spectra of apatite from different areas of the material (Figure 10), which is generally associated with bone or ivory. A subsequent microscopic examination revealed an extremely small amount of coral adhering to a base made of apatite-containing Figure 8. Small iron fibula from Ilvesheim “Weingärten”. The picture below shows the detail of the bead material (Figure8, below). However, due to poor preservation, it cannot be clarified whether a lack of on the bow with the microscopic coral piece adhering to a base made of apatite-containing material. coral had to be concealed or whether a red-white contrast was intended.

Figure 9. Ilvesheim “Weingärten”; Raman spectrum of the bow bead (see Figure 8). The

characteristic ν1 and ν2 frequencies of the pigments at 1522 and 1131 cm−1 are still very intense. Red = pigments; black = calcite.

FigureFigure 9. 9.IlvesheimIlvesheim “Weingärten”;“Weingärten”; Raman Raman spectrum spectrum of the bowof beadthe bow (see Figurebead8 ).(see The characteristicFigure 8). The ´1 −1 characteristicν1 and ν2 νfrequencies1 and ν2 frequencies of the pigments of the at pigments 1522 and 1131at 1522 cm andare 1131 still cm very intense.are still Redvery = intense. pigments; Red = pigments;black =black calcite. = calcite.

Figure 10. Ilvesheim “Weingärten”; Raman spectrum of the bow bead (see Figure 14) with apatite at 955–960 cm−1 and amorphous carbon.

Minerals 2016, 6, 56 12 of 22

Figure 8. Small iron fibula from Ilvesheim “Weingärten”. The picture below shows the detail of the bead on the bow with the microscopic coral piece adhering to a base made of apatite-containing material.

Minerals 2016, 6, 56 16 of 22 Figure 9. Ilvesheim “Weingärten”; Raman spectrum of the bow bead (see Figure 8). The Mineralscharacteristic2016, 6, 56 ν1 and ν2 frequencies of the pigments at 1522 and 1131 cm−1 are still very intense. Red 13= of 23 pigments; black = calcite.

Figure 13. Comparison of Raman spectra from a white spot and from a red part of a single branch of CoralliumFigure 10. rubrum Ilvesheim from “Weingärten”; the Mediterranean Raman Sea, spectrum all taken of with th thee bow487.9 bead nm (Ar(see + );Figure (P) stands 14)11) with for polyenesapatite at −1 and955–960 (C) for cm calcite.´ 1andand amorphous amorphous carbon. carbon.

FigureFigure 14. 11. RamanRaman spectra ofof SpondylusSpondylus gaederopus gaederopusfrom from Morea, Morea, Greece, Greece, taken taken with with 532 532 nm nm (diode). (diode). Left: Left: white interior of shell with aragonite bands at 154, 207, 706 and a narrow2´ (CO3)2− peak át1 white interior of shell with aragonite bands at 154, 207, 706 and a narrow (CO3) peak at 1085 cm , 1085but nocm polyenes.−1, but no Right:polyenes. colored Right: exterior colored showing exterior the showing polyene mainthe polyene bands at main 1125 bands and 1511 at 1125 cm´ 1andand 1511several cm combinations−1 and several and combinations overtone bands, and asovertone well as calcitebands, featuresas well atas 275 calcite and 713features cm´1 .at 275 and 713 cm−1. 3.4. Identification Methodology A comparison of the spectra from colored and uncolored corals and mollusks (Figures 13 and 14) suggestsMethodologically, that the main bandwidth these previous of CaCO findings3 at 1086 on cm the−1 structureis smaller, and when color no ofband archaeological characteristic and of therecent colorantC. rubrum is detected.artifacts In andaddition, other the biominerals CaCO3 band (see of [54 mollusks,]) can be and joined especially together of in sea the snails form at of arounda non-destructive 1086 cm−1, seems multi-stage to be approachusually sharper to identify than redthe co coralsrresponding and light band colored of corals, biominerals even if withthe shellsemphasis are also on colored. optical examination, The larger carbonate digital microscopyband in corals and may Raman be caused spectroscopy by the pa withrtial substitution a laboratory (lessinstrument than 10 or mol a portable %) of Mg one.2+ for Ca2+ in the calcite, causing a static positional disorder, as described by UrmosAs Figure et al. [18]. 12 illustrates, the fastest and easiest way to identify C. rubrum is the detection of characteristic structures by optical examination. In most cases, the surface shows significant features of Minerals 2016, 6, 56 13 of 22

3.4. Identification Methodology Methodologically, these previous findings on the structure and color of archaeological and recent C. rubrum artifacts and other biominerals (see [54]) can be joined together in the form of a non-destructive multi-stage approach to identify red corals and light colored biominerals with emphasis on optical examination, digital microscopy and Raman spectroscopy with a laboratory instrument or a portable one. Minerals 2016, 6, 56 14 of 23 As Figure 11 illustrates, the fastest and easiest way to identify C. rubrum is the detection of characteristic structures by optical examination. In most cases, the surface shows significant features ofone one or theor otherthe other material. material. As already As already mentioned mentione above,d theabove, most the characteristic most characteristic structure structure of C. rubrum of C.is therubrum longitudinal is the longitudinal crenulation crenulation of the surface of the that surfac cane also that be can seen also in be the seen perpendicular in the perpendicular section as sectionconcentric as concentric rings around rings the around medullar the regionmedullar (see region Figure (see3). Figure 3).

FigureFigure 11. 12. ProtocolProtocol for for non-invasive non-invasive identification identiﬁcation of of CoralliumCorallium rubrum ..

IfIf an an optical investigation fails, fails, the the identification identification of of organic organic pigments pigments and and calcite calcite by by Raman spectroscopyspectroscopy limits limits the the group group of of possible possible materials materials from from the the adjacent adjacent seas seas thatthat were were accessible accessible at that at timethat timeto C. to rubrumC. rubrum andand the thecolored colored parts parts of ofsome some big big shells, shells, like like SpondylusSpondylus. .Since Since Octocorals, Octocorals, like like C. rubrumrubrum,, consistconsist of of an an HMC, HMC, unlike unlike mollusks, mollusks, this shiftthis couldshift (theoretically)could (theoretically) function function as an indicator as an indicatorfor material for identificationmaterial identification (see the discussion (see the discussion below). below). AsAs a finalfinal step,step, if if a a patina patina of of metal metal oxide oxide covers cove thers surfacethe surface or the or material the material does notdoes allow not aallow Raman a Ramanspectroscopic spectroscopic investigation, investigation, complementary complementary mobile analytical mobile techniquesanalytical techniques can be used. can The be combination used. The combinationof XRF and XRDof XRF together and XRD with together environmental with environm scanningental electron scanning microscopy electron microscopy (ESEM) or (ESEM) micro-CT or micro-CTenables the enables recognition the ofrecognition the (micro-)structural of the (micro and-)structural chemical features and chemical of even strongly-fluorescent features of even strongly-fluorescentobjects. ESEM is a non-invasive objects. ESEM technique, is a non-invasive which allows technique, the observation which allows of the microstructural the observation features of the microstructuralpreserved in the features archaeological preserved remains in andthe toarchae compareological this withremains the microstructuraland to compare data this of withCorallium the microstructuraland other organisms. data of Corallium and other organisms.

3.5. Summarized Results Besides the recent and archaeological reference objects, 72 artifacts, decorated with a total of 263 pieces of light colored biominerals, were optically and Raman spectroscopically investigated in order to determine the used materials. It has to be noted that the evaluation of Table2 is not statistically significant. The high number of 45 unidentified pieces due to patina derives from the analysis of only four artifacts from one Minerals 2016, 6, 56 15 of 23 location. Among these objects, one fibula was ornamented with 109 pieces. If these pieces had not been analyzed, the proportion of red coral after investigation would have risen to 77.5%. Beside these statistical notes, the results demonstrate the need for further studies: a correction of the material reported in the literature or at least a clarification for uncertain materials could be made for 25 out of 72 archaeological objects with light colored inlays (for an overview, including archaeological reference objects, see Table S1).

Table 2. Non-representative list of investigated decorative elements showing the previously-supposed material, the speciﬁc result of our examination and, ﬁnally, the interpretation of the data in terms of the most probable original material. The data consist of Raman spectroscopic and optical investigations.

Number of Pieces 1 Previous Material Status New Material 169 Red coral approved Red coral Proportion of approved materials: 64.3% 5 Previously unspecified clarification Red coral 1 Amber or coral clarification Red coral 1 Not specified clarification Enamel 4 Amber or coral clarification Not coral, possibly amber Proportion of clarifications: 4.1% partial specification 5 Red coral I.g. some kind of carbonate of substance Proportion of partial specifications 2: 1.9% 1 “White paste” different material Red coral 2 Bone different material Red coral 2 Bronze different material Red coral 7 Red coral different material White coral or shell 1 Amber different material White coral or shell 3 Red coral different material Ivory (or bone) 4 Red coral different material Enamel 7 Red coral different material Unidentified material Proportion of changed material interpretations: 10.3% No results due 45 Red coral Unknown to patina Proportion of pieces without result due to patina 3: 17.1% Red coral, not No signal, only 6 Unknown specified, bone fluorescence Proportion of pieces without result due to fluorescence: 2.3% Total: 263 Proportion of red coral after investigations: 68.4% 1 “Pieces” refers to the single segments of a coral ornament, which can number several hundred for one single object, like a helmet or fibula; 2 red coral was supposed, but the quality of the spectra was too poor to positively approve it; 3 they originate from one single location with one fibula, decorated with 109 supposed coral pieces.

4. Discussion Although an attribution to or an exclusion of C. rubrum is successful in most cases, some challenges in terms of material determination need to be addressed in order to indicate the scope of further studies.

4.1. Challenges of Optical Identiﬁcation Archaeological biominerals are subjected to diagenetic alterations, which can hamper the identiﬁcation of red coral on archaeological artifacts. Archaeological shells may show a lamellar surface structure that resembles that of C. rubrum. Especially for small pieces like ground or polished Minerals 2016, 6, 56 15 of 22

4.1. Challenges of Optical Identification

MineralsArchaeological2016, 6, 56 biominerals are subjected to diagenetic alterations, which can hamper 16the of 23 identification of red coral on archaeological artifacts. Archaeological shells may show a lamellar surface structure that resembles that of C. rubrum. Especially for small pieces like ground or polished studsstuds displaying displaying such such surface features,features, aa distinction distinction between between mollusks mollusks and and corals corals based based on the on surface the surfacemorphology morphology may be verymay difﬁcult, be very as thedifficult, detail of theas Ironthe Agedetail shell of from the Saint-Hilaire-le-Grand Iron Age shell from in the Saint-Hilaire-le-GrandChampagne region in in France the Champagne shows in Figure region 13 in (see France also Figureshows3 ine). Figure However, 12 (see in comparison also Figure to3e). the However,coral pieces, in comparison mollusks can to the still coral have pieces, characteristically mollusks can shiny still nacreoushave characteristically parts. Furthermore, shiny nacreous the typical parts.layered Furthermore, shell structure the typical differs fromlayered those shell of corals,structure because differs these from layers those are ofnot corals, spherical, because but these rather layershorizontal are not and spherical, have a much but rather larger horizontal diameter compared and have to a themuch crenulated larger diameter surface of comparedC. rubrum to. the crenulated surface of C. rubrum.

(a) (b)

FigureFigure 12. 13. IronIron Age Age shell shell from from Saint-Hilaire-le-Gra Saint-Hilaire-le-Grandnd in inthe the Champagne Champagne region region in inFrance France (a) ( aand) and a a microscopicmicroscopic image image (b ()b of) ofthe the structural structural features. features.

4.2.4.2. Possibilities Possibilities and and Limitations Limitations of ofRaman Raman Spec Spectroscopytroscopy for for the the Identification Identification of ofC. C. rubrum rubrum InIn most most cases, cases, Raman Raman spectroscopy spectroscopy yields yields good good results results to todistinguish distinguish between between different different biominerals,biominerals, but but it itis isstill still very very hard hard or or almost almost impossible impossible at atthe the moment moment to todifferentiate differentiate between between calciticcalcitic colored colored shells shells and and C.C. rubrum rubrum. . FigureFigure 13 14 illustrates illustrates the the direct direct correlation correlation be betweentween color color and and the the intensity intensity of ofRaman Raman bands bands associated with the red pigment. Measurements of a small white spot on an otherwise red piece of associated with the red pigment. Measurements of a small white spot on an otherwise red piece of C. rubrum from the Mediterranean Sea with a 487.9-nm Ar+-laser excitation, which is particularly C. rubrum from the Mediterranean Sea with a 487.9-nm Ar+-laser excitation, which is particularly sensitive to colorants, obtained almost no indications of polyenes at around 1130 and 1520 cm−1, sensitive to colorants, obtained almost no indications of polyenes at around 1130 and 1520 cm´1, while while the surrounding red area yielded a typical spectrum for C. rubrum with intense polyene bands the surrounding red area yielded a typical spectrum for C. rubrum with intense polyene bands and a and a small one for (CO2´3)2− at about 1086 cm´1 −1. The HMC features in the lower frequency region were small one for (CO3) at about 1086 cm . The HMC features in the lower frequency region were too too small to be observed by 488-nm excitation without clipping. The Raman spectrum for the light small to be observed by 488-nm excitation without clipping. The Raman spectrum for the light red red colored periphery of the white spot shows very small bands at 1134 and 1526 cm−1, characteristic colored periphery of the white spot shows very small bands at 1134 and 1526 cm´1, characteristic of of the organic colorant. the organic colorant. The same effect can be observed on shells (Figure 14), where the inner side of the shell is mostly The same effect can be observed on shells (Figure 11), where the inner side of the shell is mostly white and yields a spectrum of almost pure aragonite with bands at 154, 207, 706 and 1085 cm−1, white and yields a spectrum of almost pure aragonite with bands at 154, 207, 706 and 1085 cm´1, respectively; Eg,ext modes for the two first bands (T, L) and Eg,int (ν4) and A1g,int (ν1) modes for the respectively; Eg,ext modes for the two first bands (T, L) and Eg,int (ν4) and A1g,int (ν1) modes for the following. In contrast, the spectrum of the purple colored exterior shows calcitic features at 275 and following. In contrast, the spectrum of the purple colored exterior shows calcitic features at 275 and −1 −1 713 cm .´ The1 ν1 and ν2 frequencies at 1511 and 1125 cm óf1 the colorants are so intense that the 713 cm . The ν1 and ν2 frequencies at 1511 and 1125 cm of the colorants are so intense that the 2− (CO3) band2´ is superimposed. Furthermore, several combinations and overtone bands are visible. (CO3) band is superimposed. Furthermore, several combinations and overtone bands are visible. A comparison of the spectra from colored and uncolored corals and mollusks (Figures 11 and 14) ´1 suggests that the main bandwidth of CaCO3 at 1086 cm is smaller, when no band characteristic of the colorant is detected. In addition, the CaCO3 band of mollusks, and especially of sea snails at around 1086 cm´1, seems to be usually sharper than the corresponding band of corals, even if the shells are also colored. The larger carbonate band in corals may be caused by the partial substitution (less than 10 mol %) of Mg2+ for Ca2+ in the calcite, causing a static positional disorder, as described by Urmos et al. [18]. Minerals 2016, 6, 56 17 of 23 Minerals 2016, 6, 56 16 of 22

Figure 13. 14. Comparison of Raman spectra from a white spot and from a red part of a single branch of Corallium rubrum from the Mediterranean Sea, all taken withwith 487.9487.9 nmnm (Ar(Ar++);); (P) stands for polyenes and (C) for calcite.

The identification of unknown archaeological biominerals can be hampered due to their high fluorescence and poor signal to noise ratio. However, the Raman spectra of only six of a total of 263 analyzed decoration materials could not be evaluated due to high fluorescence. As this corresponds to a very low proportion of the analyzed artifacts (only 2.3%), special methods to reduce the fluorescence problem were not applied. Thanks to the resonant Raman obtained with coral colorants using specific laser wavelength, its detection is achievable over the fluorescence signal even with the very low remaining contents. Further studies will allow determining if for these six artifacts, the absence of a Raman signature from the colorant is due to deep degradation or the use of another material than coral.

4.3. Hypotheses for the Loss of Color of Archaeological Corals The loss of color of archaeological corals is probably linked to diagenetic alterations occurring over time due to various processes, depending on the environmental conditions (temperature, humidity, acidity and composition of the soil, activity of microorganisms), which were very well studied for other biominerals, like bones or teeth (e.g., [55]). This raises the question of where the colorants are actually located. To our knowledge, localization of colorants has not yet been explored in detail. Systematic optical examination of ancient corals allows several assumptions about the encapsulation of the polyenes within the axial skeleton and the process of bleaching. A ﬁrst hypothesis addresses the organic diagenesis. As demonstrated by Figures3a and4, the pieces that are today light colored were originally as red as modern precious corals. The loss of color usually begins at the most exposed point of the piece and initially spreads along the surface until it penetratesFigure into14. Raman the internal spectra structure of Spondylus of the gaederopus object. Thefrom characteristic Morea, Greece, crenulations taken with 532 of the nm growth (diode).rings on theLeft: perpendicular white interior sections of shell ofwith archaeological aragonite bands corals at appear154, 207, very 706 oftenand a andnarrow look (CO like3) the2− peak concentric at −1 growth1085 rings cm , of but an no annular polyenes. domain Right: with colored stained exterior organic showing matter the polyene (OM), as main in Figure bands3 atg. 1125 However, and in −1 contrast1511 tocm modern and several stained combinations coral, the archaeological and overtone piecesbands, doas notwell onlyas calcite exhibit features color differencesat 275 and due −1 to partial713 cm fading,. like on the Somme-Tourbe piece (Figure3f), sometimes even small gaps between the concentric rings appear, as seen on the Magdalenenberg piece (Figure3e). Vielzeuf et al. [13] have A comparison of the spectra from colored and uncolored corals and mollusks (Figures 13 and 14) shown that “the growth rings are revealed by the cyclic variation of organic matter (OM) and Mg/Ca suggests that the main bandwidth of CaCO3 at 1086 cm−1 is smaller, when no band characteristic of ratio”. The loss of color seems to be accompanied by a change in ultrastructure, which leads us to the the colorant is detected. In addition, the CaCO3 band of mollusks, and especially of sea snails at hypothesis that these gaps, which are normally not visible on modern corals without any treatment, around 1086 cm−1, seems to be usually sharper than the corresponding band of corals, even if the may be caused by the decay of the OM by time, resulting in a ﬁssured structure in the shape of the shells are also colored. The larger carbonate band in corals may be caused by the partial substitution (less than 10 mol %) of Mg2+ for Ca2+ in the calcite, causing a static positional disorder, as described by Urmos et al. [18].

Minerals 2016, 6, 56 18 of 23 modern corals after coloration treatment. Nevertheless, there still is enough colorant left, perhaps in the sclerites, to obtain Raman bands under resonant conditions. As a further hypothesis, the loss of color may be related to the diagenetic processes of the HMC. Concerning coral diagenesis, some work was done on aragonitic corals to study the effect of diagenesis on paleoclimate reconstruction [56]. Dissolution of primary coral aragonite, inﬁlling of skeletal pores and recrystallization of aragonite to calcite were observed. For calcitic corals, dissolution of HMC (leaching of Mg) and stabilization as LMC is thinkable. The bleaching could also be related to this process. Finally, human activities, such as an intentional bleaching, e.g., by boiling, could be the cause. However, this hypothesis appears to be not very resilient. First, it seems rather unreasonable to import corals over such a long distance only to change the color until they look like ordinary bone or tooth, especially not at a time when vivid colors were rare. Secondly, there are sometimes still red areas visible on the front side of the inlays (see Figure3f), mostly on the edges of inlays, next to the surrounding metal, which seems to have prevented fading. Red areas around rivets, as seen in Figure3h, can be observed very often, as well. Metals can have an antibacterial effect (e.g., Cu in bronze). The authors have observed an inﬂuence of the presence of heavy metals, including copper, on the degradation of the microstructure of archaeological bone samples due to microbial activity [57]. Future works aim at a better understanding of the interaction of the organic matrix or the mineral part with the coloring components of the corals, of diagenetic alterations of archaeological corals in order to understand the alteration processes resulting in a loss of color and a change of the surface structure.

4.4. Archaeological Implications We are aware of the fact that our method would not sufficiently meet the standards for the identification of marine ecology, Earth science, etc. However, for Iron Age research, where it can be assumed from the archaeological record that the marine material most probably came from the Mediterranean Sea or from the Atlantic and the Northern seaboard, the range of potential raw material is considerably smaller, and many other species can be ruled out by the simple fact that they do not occur near the European coastline. The analyses of the beads on the fibula from Schwieberdingen (see Figure5) demonstrate that different color accents were used to decorate one single object. Prior to these studies allowing the detection of colorants in already faded biomineral, it has been a matter of controversy whether only red colored corals were used for Iron Age ornaments or if also light colored materials were employed and to what extent (e.g., [58–61]). Furthermore, the proof of C. rubrum for this fibula changes the interpretation of the whole burial. Besides the small gold foil, the corals (there are two more fibulae with light colored beads) are the only precious material among the grave goods, and they document connections to Mediterranean trade networks. These two aspects in turn raise the status of the buried person. From the archaeological point of view, the identification of the used material is very important because the evidence of its presence may change the whole interpretation of an archaeological object or site. In comparison to the trade route models derived for other Mediterranean goods, the distribution pattern of (supposed) coral finds appears to be more dynamic, indicating trade routes in a higher spatial resolution and distributional “hot spots”. They also play a crucial role in the reconstruction of social structures. As an indicator of prestige, the proof of coral in a grave shifts the notional social position of the deceased. Furthermore, there has been a dispute if red corals were used together with red glass, which became popular during the third century BC, to decorate objects. Since the corals appear almost white today and it would not make much sense to use two different red materials, it was assumed that they were white in the first place. Our proof of polyenes in the case of the fibula of Gäufelden-Nebringen (Landkreis Böblingen), Grave 17 (see Table S1), argues against this assumption. Minerals 2016, 6, 56 19 of 23

5. Conclusions This study showed that analytical examinations of light colored biominerals can provide key information on the inlay materials of Iron Age precious objects. A combination of optical investigations and Raman spectroscopy allow a reliable identification of Corallium rubrum with respect to bone and ivory. Because Raman spectroscopy is non-invasive, requiring only a low amount of material without sampling or sample preparation, and allowing short measurement times, it is a very efficient method, especially for analyzing archaeological materials and/or gemstones [62,63]. In addition, thanks to the availability of portable instruments, measurements can be performed on site in museums. Foremost, it enables a limitation or even identification of the most eligible materials. Raman spectroscopy possesses a special benefit for the study of colored biominerals. Based on the study of 95 archaeological artifacts and 41 recent corals and mollusks, the non-invasive identification procedure was established and successfully tested. The distinction between C. rubrum and the calcitic outer part of a colored Spondylus shell by Raman spectroscopy can sometimes be effected on the basis of Mg contents (HMC in C. rubrum and LMC in Spondylus), and the smaller bandwidth of the ν1 carbonate band of mollusks at around 1086 cm´1 can serve as an indication. However, due to the substitution of Mg by time, this feature is not fully reliable. If also an optical investigation yields no characteristic structural features because of a processed surface, the identification procedure needs to be completed by additional analyses by XRF, XRD, ESEM or micro-CT that allow the determination of the MgCO3 content and the microstructure of the material. Nevertheless, much new information of archaeological relevance can be gained from a systematic study of Iron Age inlays using the newly-established identification procedure. This study reveals a previously unsuspected variety of materials and colors. This changes our picture of relatively uniform costumes to a more creative and individual style, possibly depending on the available resources. In the context of the whole grave or, rather, the deceased, the identification of a variety of employed materials helps to refine the scales of social distinction by conspicuous consumption of prestige goods. A large fibula, but with inlays made from ordinary bone, for example, may not have been as valuable as a smaller one, but with exotic red corals. In times when bright colors usually were rare, corals must have been like signal lights of social status. Finally, on a macro-sociological level, a reliable approximation of the extent of coral use enables a mapping of trade routes and centers of distribution and consumption to gain a better understanding of the trade, communication and social structure of the Iron Age. The same kind of procedure can also be adapted to other equivalent biominerals of archaeological relevance. Therefore, the full potential of the combination of the field of biomineral and biomineralization study and archaeology still needs to be exploited. Many new interesting results are expected from such studies to provide further information on the use of these materials in past societies and ancient ways of life. Furthermore, future research work is planned focusing on a better understanding of the bleaching processes in archaeological red corals. This requires an improved knowledge of the colorant nature and its interaction with the coral organic and mineral part, as well as an in-depth study of diagenetic changes occurring in archaeological corals over time.

Supplementary Materials: The following are available online at www.mdpi.com/2075-163X/6/2/56/s1, Table S1: List of objects from archaeological contexts that have been analyzed by Raman spectroscopy, Table S2: List of used Raman spectroscopes with their settings and the amount of measurements. Acknowledgments: The authors wish to thank the reviewers and the editor for their insightful comments and suggestions, as well as Daniel Vielzeuf (CINaM (Centre Interdisciplinaire de Nanoscience de Marseille)) for his friendly instructions. Furthermore, our thanks go to the following museums and research institutes for their valuable contribution to this work by providing their objects: Archäologisches Landesmuseum Baden-Württemberg, Zentrales Fundarchiv Rastatt, Badisches Landesmuseum Karlsruhe, Institute for Gemstone Research Mainz and Idar-Oberstein, Landesmuseum Mainz, Landesmuseum Württemberg Stuttgart, Musée d’Archéologie Nationale Saint-Germain-en-Laye, Museum der Stadt Worms Worms im Andreasstift, Natural History Museum Mainz, private Collection Klaus Paysan, Reiss-Engelhorn Museen Mannheim, Minerals 2016, 6, 56 20 of 23

Romano-Germanic Central Museum in Mainz, Senckenberg Research Institute Frankfurt/Main, Museum im Gelben Haus in Esslingen, University of Mainz. The Institute of Geosciences at the University of Mainz is thanked for providing parts of the equipment. Author Contributions: Sebastian Fürst, Christopher F.E. Pare and Ina Reiche conceived of and designed the experiments. Ludovic Bellot-Gurlet, Katharina Müller, Céline Paris, Liliana Gianni and Ina Reiche performed the experiments. Liliana Gianni and Katharina Müller analyzed the data. Sebastian Fürst, Katharina Müller and Ina Reiche wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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